Optimize fortran loops with +-1 step.

Optimize fortran loops with +-1 step.

	* gfortran.dg/do_1.f90: Remove a corner case that triggers
	an undefined behavior.
	* gfortran.dg/do_3.F90: Likewise.
	* gfortran.dg/do_check_11.f90: New test.
	* gfortran.dg/do_check_12.f90: New test.
	* gfortran.dg/do_corner_warn.f90: New test.
	* lang.opt (Wundefined-do-loop): New option.
        * resolve.c (gfc_resolve_iterator): Warn for Wundefined-do-loop.
	(gfc_trans_simple_do): Generate a c-style loop.
	(gfc_trans_do): Fix GNU coding style.
	* invoke.texi: Mention the new warning.

From-SVN: r238114